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Plain Talk About Soil Chemistry
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Plain Talk About Soil Chemistry



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  • Shopping for cereal with a two year old. The example of grass containing protein – but we don’t want it, and cattle don’t eat steaks Microbiology, pH and Temperature impact nutrient forms
  • Soils have negative charge Nutrients have positive or negative charge You might have heard these terms
  • I learned that not everyone remembers “opposites attract,” so it is good to say it. Might also mention that e.g., K+ is an important plant nutrient?
  • Cation Exchange is trading: Plants trade what they cannot use for what they can. The soil acts as like the old drive-in movie theater.
  • Soil pH influences plant nutrient availability. The chemistry behind soil pH is well understood, liming produces predictable responses
  • Note that the greatest number of nutrients are non-limiting around pH 6 to 6.5. When bars touch top of their “band” (e.g., manganese, aluminum, iron, zinc at low pH and molybdenum at high pH), the element is available to the extent it becomes TOXIC. The low dip (e.g., in potassium around pH 8) means maximal availability of a “competing” ion OR binding by the mentioned ion (calcium+P = apatite, the stuff that makes up bones. VERY insoluble!) makes the ion in question (potassium/phosphorus) limiting. Call out Boron for the growers. Plants with really high nutrient requirements grow best at the middle pH Microbial activity impacted at xtremely lo and hi


  • 1. Plain Talk About Soil Chemistry Linda J. Brewer College of Agriculture Oregon State University
  • 2. Fact #1: Plant nutrients must be dissolved in water.
  • 3. Fact #2: Dissolve-ability impacts availability.
    • Some nutrients easily dissolve in water.
    • Some nutrients take years to dissolve.
    • Application timing can be your ally –     or your enemy.
  • 4. Fact #3: Roots are fussy shoppers.
    • Roots recognize only specific nutrient forms.
    • Nitrogen is the only nutrient plants can recognize in each of two forms.
  • 5. Fact #4: Ammonia and ammonium fertilizers acidify soil.
  • 6. Fact #5:Like diamonds, clay soils are forever.
  • 7. Pop Quiz: What is the charge on soils?
  • 8. Fact #6: Soils have a negative charge.
  • 9. Fact #7: Attraction is everything in fertility management.
    • Nutrient management is like teenagers in love.
    • Timing determines nutrient use.
    • Cations = + charge
    • Anions = - charge
    • Cation Exchange Capacity
  • 10.  
  • 11. Cation Exchange is a Swap Meet. H + Al +3 Ca +2 Ca +2 Mg +2 K + K + H + H + H + H + Plant root Soil
  • 12. Fact #8: Lime comes in 2 flavors
    • Agricultural lime – calcium
    • Dolomitic lime – calcium + magnesium
    • It’s the carbonate that changes pH, not the calcium or the magnesium!
    • Gypsum – CaSO 4 – provides calcium but does not change pH.
  • 13. Fact #9: Right soil pH unlocks nutrient availability.
    • pH controls nutrient form and dissolve-ability.
    • Growers can impact soil pH.
  • 14. Soil pH & Nutrient Availability
  • 15. Fact #10: Home soil test kits are OK . . . maybe
    • The results ain’t necessarily so
    • You don’t need to be a rocket scientist to recognize a problem
  • 16. Summing it all up:
    • Nutrients must be dissolved in water.
    • Dissolve-ability impacts availability.
    • Roots are fussy shoppers.
    • Soil has a negative charge.
    • NH 3 and NH 4 + acidify soil.
    • Soil pH controls nutrient form.
    • Home soil test kits.
  • 17. Linda J. Brewer Faculty Research Assistant Oregon State University College of Agriculture 541-737-1408 [email_address]